The invention relates to a heat recovery module for a water heater.
In known storage-type water heaters, water in a holding tank is heated during standby such that the water is of a suitable temperature during draws. The water expands as it is heated, which causes some of the water to be displaced out of the tank and into the cold water supply pipe. Thermal energy in the displaced water is lost as the displaced water cools in the cold water supply pipe.
One solution to the loss of thermal energy during displacement is to insulate a length of the cold water supply pipe. This solution has its limitations, however. Because of the relatively small diameter of most cold water supply pipes, the surface area-to-volume factor or ratio of the cold water supply pipe is relatively large (on the order of 5 inches squared/inches cubed). A large surface area-to-volume factor hinders the effective insulation of the pipe. Also, cold water supply pipes typically extend vertically from the water heater, and therefore facilitate thermal transfer from the hot water at the bottom of the pipe to the cold water in the pipe under the influence of natural convection currents, even if the pipe is insulated. Also, in a typical ten gallon draw, the amount of water that will be forced from the tank due to the subsequent thermal expansion is equivalent to approximately one pint. In the case of a xc2xe inch cold water supply pipe, displacement of one pint of water would require the insulation of more than five feet of the cold water supply pipe.
The invention provides a water heater comprising a storage tank for holding water, a heating device for heating the water in the tank, a hot water outlet communicating between the tank and a hot water supply pipe, and a cold water inlet communicating between the tank and a cold water supply pipe. Preferably, the inlet includes a dip tube extending to the lower portion of the tank and communicating with the cold water supply pipe. The invention also provides a heat recovery module having an interior cavity that communicates between the dip tube and the cold water supply pipe. The heat recovery module preferably includes insulation surrounding the cavity. The surface area-to-volume ratio of the cavity is smaller than the surface area-to-volume ratio of the cold water supply pipe.
As cold water is heated within the tank, the water expands and forces some of the water out of the tank and into the cavity of the heat recovery module. Water in the cavity naturally stratifies such that the warmest water rises to the upper portion of the cavity and the coldest water sinks to the lower portion of the cavity. The cold water supply pipe communicates with the lower portion of the cavity such that, as the water expands into the heat recovery module, only the coldest water in the cavity is displaced into the cold water supply pipe.